1. Field of the Invention
The present invention relates to a digital camera for performing automatic focus adjustment of a phase detection method.
2. Description Related to the Prior Art
Most digital cameras comprise an autofocus device (AF device) for performing automatic focus adjustment. The AF device checks an image forming state of a taking lens and moves a focus lens, which is a part or the entire of the taking lens, to clearly form an image of a main subject on an imaging surface of a solid-state imaging device such as a CCD or a CMOS.
There are AF devices of contrast detection method and AF devices of phase detection method. In the contrast detection method, a lens position for maximizing an integrated value of differences between signals from adjacent pixels is determined as an in-focus position while the focus lens is moved in an optical axis direction. In the phase detection method, the in-focus position is obtained by detecting the presence or absence of a shift between incident positions of light passing through different portions of a taking lens and the magnitude of the shift. The contrast detection method is inexpensive because the focus is detected using a pixel signal obtained from the solid-state imaging device. The phase detection method allows fast AF control because a focus state is detected without moving the focus lens. However, with the need of a phase detection sensor, the phase detection method tends to be employed by expensive models.
For example, U.S. Patent Application Publication No. 2012/0033120 (corresponding to Japanese Patent Laid-Open Publication No. 2010-252277) and Japanese Patent Laid-Open Publication No. 2007-158597 disclose digital cameras comprising an AF device of a phase detection method. A solid-state imaging device provided with two types of phase difference pixels (first and second phase difference pixels) in addition to normal pixels, thereby eliminating the need for a special phase detection sensor, is also known (see U.S. Pat. No. 8,466,998 (corresponding to Japanese Patent Laid-Open Publication No. 2008-312073), Japanese Patent Laid-Open Publication No. 2009-157198, and Japanese Patent Laid-Open Publication No. 2009-017152). The normal pixel has a normal opening section located above the center of a light receiving surface of a photodiode. The normal pixel photoelectrically converts light incident thereon through the normal opening section. The first phase difference pixel has a first off-center opening section that is shifted in a first direction relative to the center position of a light receiving surface of a photodiode. The second phase difference pixel has a second off-center opening section that is shifted in a second direction opposite to the first direction.
The normal opening sections, the first off-center opening sections, and the second off-center opening sections are formed through a light-shield film for covering a semiconductor substrate in which the photodiodes are formed. The size of each of the first and second off-center opening sections is smaller than the size of the normal opening section.
The phase difference pixel is highly sensitive to light incident from an oblique direction relative to a center line of a pixel. For example, the first phase difference pixel is highly sensitive to the light incident from upper right. The second phase difference pixel is highly sensitive to light incident from upper left.
An image obtained from the first phase difference pixels and an image obtained from the second phase difference pixels are shifted in a right-left direction in accordance with a focus state of the taking lens. An amount of shift between the two images corresponds to an amount of shift of the focus of the taking lens. The two images coincide and the amount of shift is zero when the taking lens is in focus. The amount of shift increases as the taking lens is further out of focus. Hence, a focus adjustment amount of the taking lens is calculated by detecting the direction of the shift of each image and the amount of shift between the images obtained from the first and second phase difference pixels.
In the solid-state imaging device disclosed in the U.S. Pat. No. 8,466,998, the Japanese Patent Laid-Open Publication No. 2009-157198, and the Japanese Patent Laid-Open Publication No. 2009-017152, color filters are provided for the first and second phase difference pixels and the normal pixels. The digital camera comprising the solid-state imaging device uses all of the normal pixels, the first phase difference pixels, and the second phase difference pixels to capture a normal image.
In the above-described solid-state imaging device, the first and the second phase difference pixels have the color filters. Hence, output levels from the first and second phase difference pixels significantly decrease depending on a color of a subject. As a result, the focus adjustment amount cannot be obtained accurately. For example, in a case where the first and the second phase difference pixels have color filters of three colors, red, green, and blue, output levels of the first and second phase difference pixels with the green and the blue color filters are low when an image of a red subject is captured.